It is known that arterial compliance which is the variation in the cross-section of the artery for a corresponding variation of the pressure reflects the elastic behaviour of the artery. This compliance is considered as indispensable to good knowledge of the physiology, the physiopathology and the therapy of the arterial system. This compliance is a function of the arterial pressure and in order to establish it one thus needs the instantaneous relationship which exists between the pressure and the diameter at a given point of the artery.
Propositions for measuring the pressure-diameter relationship have already been advanced, for example in the study presented on pages 789 to 793 of the review Arch. Mal. Coeur, Nr. 6, 1987, where the visco-elastic behaviour of the aorta was analyzed in a conscious dog. The visco-elastic response of the aorta to the administration of hormones is observed in the cited study by analyzing the pressure-diameter relationship of the aorta. This relationship is established by means of a microsensor for pressure which may be calibrated in situ and introduced through the left humeral artery and placed in the light of the descending aorta and of two piezoelectric crystals of 4 mm diameter diametrally attached in the envelope of the proximal descending aorta.
The means which have just been suggested have an invasive character, i.e. they affect the integrity of the organs in which they intervene. In respect of the human body, one prefers to employ sensors enabling non-invasive measures, such sensors remaining placed at the surface of the artery to be measured without any penetration into the surrounding tissues.
Non-invasive sensors enabling the continuous measurement of blood pressure are known. In particular, there may be mentioned the photoplethysmograph sold by the Ohmeda Company, 3030 Airco Drive, Madison, Wis., USA and bearing the registered trademark "finapres" (for finger arterial pressure). As indicated, the apparatus measures the blood pressure at the end of a finger according to the method described in the article "Effects of Peripheral Vasoconstriction on the Measurement of Blood Pressure in a Finger" in the review Cardiovascular Research, 1985, 19, 139-145.
Non-invasive sensors enabling the measurement of the arterial diameter are likewise known. In particular reference is made to the apparatus employed in the patent document U.S. Pat. No. 4,370,985 which enables measurement of the diameter of the artery by sending an ultrasonic wave on to the artery and measuring the echo sent back by the artery walls. This diameter measurement may be brought about on superficial arteries, for instance the humeral artery or the radial artery.
From the brief description of presently known sensors which has just been given hereinabove, it is apparent that it is not possible to measure non-invasively the pressure in every artery other than that of the finger and the diameter of said artery at the same place in a manner such that the relationship or curve pressure-diameter exhibits a systematic hysteresis. This is due to the fact that the speed of propagation of the pressure wave being finite, the variations of pressure measured downstream exhibit a certain delay relative to the corresponding variations of the diameter. This delay is evidently more substantial when the distance which separates the two measurement sites is increased.